116 G. H. Williams — Pei/rograghical Microscope. 



•mirror-bar. It was selected in order to reduce the total ex- 

 pense as much as possible, but all the petrographical appliances 

 may be adapted to any of the brass stands of this firm, if de- 

 sired, with a proportionate increase in expense. 



(One-third natural size 



The screw supporting the arm between the pillars allows the 

 instrument to be inclined at any angle. The main tube is pro- 

 vided with a cloth lining into which the draw-tube carrying 

 the ocular, is fitted. There is a coarse adjustment by rack and 

 pinion and a fine adjustment by a micrometer screw. The 

 mirror is both flat and concave and the mirror-bar adjustable. 



Coming now to the peculiarly petrographical features, we 

 have the lower nicol-prism or polarizer enclosed in a cylindri- 

 cal metal box, both ends of which are protected by glass. 

 This box is capable of a complete revolution and is provided 



